Intel junk...Kernel-memory-leaking Intel processor design flaw forcesLinux, Windows redesign

Performance hits loom, other OSes need fixes

Updated A fundamental design flaw in Intel's processor chips has
forced a significant redesign of the Linux and Windows kernels
to defang the chip-level security bug.

Programmers are scrambling to overhaul the open-source Linux
kernel's virtual memory system. Meanwhile, Microsoft is expected
to publicly introduce the necessary changes to its Windows
operating system in an upcoming Patch Tuesday: these changes
were seeded to beta testers running fast-ring Windows Insider
builds in November and December.

Crucially, these updates to both Linux and Windows will incur a
performance hit on Intel products. The effects are still being
benchmarked, however we're looking at a ballpark figure of five
to 30 per cent slow down, depending on the task and the
processor model. More recent Intel chips have features – such as
PCID – to reduce the performance hit. Your mileage may vary.


The Register
?
@TheRegister
PostgreSQL SELECT 1 with the KPTI workaround for Intel CPU
vulnerability https://www.postgresql.org/message-
…

Best case: 17% slowdown
Worst case: 23%

3:58 PM - Jan 2, 2018
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Similar operating systems, such as Apple's 64-bit macOS, will
also need to be updated – the flaw is in the Intel x86-64
hardware, and it appears a microcode update can't address it. It
has to be fixed in software at the OS level, or go buy a new
processor without the design blunder.

Details of the vulnerability within Intel's silicon are under
wraps: an embargo on the specifics is due to lift early this
month, perhaps in time for Microsoft's Patch Tuesday next week.
Indeed, patches for the Linux kernel are available for all to
see but comments in the source code have been redacted to
obfuscate the issue.

However, some details of the flaw have surfaced, and so this is
what we know.

Impact
It is understood the bug is present in modern Intel processors
produced in the past decade. It allows normal user programs –
from database applications to JavaScript in web browsers – to
discern to some extent the layout or contents of protected
kernel memory areas.

The fix is to separate the kernel's memory completely from user
processes using what's called Kernel Page Table Isolation, or
KPTI. At one point, Forcefully Unmap Complete Kernel With
Interrupt Trampolines, aka ****WIT, was mulled by the Linux
kernel team, giving you an idea of how annoying this has been
for the developers.

Whenever a running program needs to do anything useful – such as
write to a file or open a network connection – it has to
temporarily hand control of the processor to the kernel to carry
out the job. To make the transition from user mode to kernel
mode and back to user mode as fast and efficient as possible,
the kernel is present in all processes' virtual memory address
spaces, although it is invisible to these programs. When the
kernel is needed, the program makes a system call, the processor
switches to kernel mode and enters the kernel. When it is done,
the CPU is told to switch back to user mode, and reenter the
process. While in user mode, the kernel's code and data remains
out of sight but present in the process's page tables.

Think of the kernel as God sitting on a cloud, looking down on
Earth. It's there, and no normal being can see it, yet they can
pray to it.

These KPTI patches move the kernel into a completely separate
address space, so it's not just invisible to a running process,
it's not even there at all. Really, this shouldn't be needed,
but clearly there is a flaw in Intel's silicon that allows
kernel access protections to be bypassed in some way.

The downside to this separation is that it is relatively
expensive, time wise, to keep switching between two separate
address spaces for every system call and for every interrupt
from the hardware. These context switches do not happen
instantly, and they force the processor to dump cached data and
reload information from memory. This increases the kernel's
overhead, and slows down the computer.

Your Intel-powered machine will run slower as a result.

How can this security hole be abused?
At best, the vulnerability could be leveraged by malware and
hackers to more easily exploit other security bugs.

At worst, the hole could be abused by programs and logged-in
users to read the contents of the kernel's memory. Suffice to
say, this is not great. The kernel's memory space is hidden from
user processes and programs because it may contain all sorts of
secrets, such as passwords, login keys, files cached from disk,
and so on. Imagine a piece of JavaScript running in a browser,
or malicious software running on a shared public cloud server,
able to sniff sensitive kernel-protected data.

Specifically, in terms of the best-case scenario, it is possible
the bug could be abused to defeat KASLR: kernel address space
layout randomization. This is a defense mechanism used by
various operating systems to place components of the kernel in
randomized locations in virtual memory. This mechanism can
thwart attempts to abuse other bugs within the kernel:
typically, exploit code – particularly return-oriented
programming exploits – relies on reusing computer instructions
in known locations in memory.

If you randomize the placing of the kernel's code in memory,
exploits can't find the internal gadgets they need to fully
compromise a system. The processor flaw could be potentially
exploited to figure out where in memory the kernel has
positioned its data and code, hence the flurry of software
patching.

However, it may be that the vulnerability in Intel's chips is
worse than the above mitigation bypass. In an email to the Linux
kernel mailing list over Christmas, AMD said it is not affected.
The wording of that message, though, rather gives the game away
as to what the underlying cockup is:

AMD processors are not subject to the types of attacks that the
kernel page table isolation feature protects against. The AMD
microarchitecture does not allow memory references, including
speculative references, that access higher privileged data when
running in a lesser privileged mode when that access would
result in a page fault.

A key word here is "speculative." Modern processors, like
Intel's, perform speculative execution. In order to keep their
internal pipelines primed with instructions to obey, the CPU
cores try their best to guess what code is going to be run next,
fetch it, and execute it.

It appears, from what AMD software engineer Tom Lendacky was
suggesting above, that Intel's CPUs speculatively execute code
potentially without performing security checks. It seems it may
be possible to craft software in such a way that the processor
starts executing an instruction that would normally be blocked –
such as reading kernel memory from user mode – and completes
that instruction before the privilege level check occurs.

That would allow ring-3-level user code to read ring-0-level
kernel data. And that is not good.

The specifics of the vulnerability have yet to be confirmed, but
consider this: the changes to Linux and Windows are significant
and are being pushed out at high speed. That suggests it's more
serious than a KASLR bypass.

Also, the updates to separate kernel and user address spaces on
Linux are based on a set of fixes dubbed the KAISER patches,
which were created by eggheads at Graz University of Technology
in Austria. These boffins discovered [PDF] it was possible to
defeat KASLR by extracting memory layout information from the
kernel in a side-channel attack on the CPU's virtual memory
system. The team proposed splitting kernel and user spaces to
prevent this information leak, and their research sparked this
round of patching.

Their work was reviewed by Anders Fogh, who wrote this
interesting blog post in July. That article described his
attempts to read kernel memory from user mode by abusing
speculative execution. Although Fogh was unable to come up with
any working proof-of-concept code, he noted:

My results demonstrate that speculative execution does indeed
continue despite violations of the isolation between kernel mode
and user mode.

It appears the KAISER work is related to Fogh's research, and as
well as developing a practical means to break KASLR by abusing
virtual memory layouts, the team may have somehow proved Fogh
right – that speculative execution on Intel x86 chips can be
exploited to access kernel memory.

Shared systems
The bug will impact big-name cloud computing environments
including Amazon EC2, Microsoft Azure, and Google Compute
Engine, said a software developer blogging as Python Sweetness
in this heavily shared and tweeted article on Monday:

There is presently an embargoed security bug impacting
apparently all contemporary [Intel] CPU architectures that
implement virtual memory, requiring hardware changes to fully
resolve. Urgent development of a software mitigation is being
done in the open and recently landed in the Linux kernel, and a
similar mitigation began appearing in NT kernels in November. In
the worst case the software fix causes huge slowdowns in typical
workloads.

There are hints the attack impacts common virtualisation
environments including Amazon EC2 and Google Compute Engine...

Microsoft's Azure cloud – which runs a lot of Linux as well as
Windows – will undergo maintenance and reboots on January 10,
presumably to roll out the above fixes.

Amazon Web Services also warned customers via email to expect a
major security update to land on Friday this week, without going
into details.

There were rumors of a severe hypervisor bug – possibly in Xen –
doing the rounds at the end of 2017. It may be that this
hardware flaw is that rumored bug: that hypervisors can be
attacked via this kernel memory access cockup, and thus need to
be patched, forcing a mass restart of guest virtual machines.

A spokesperson for Intel was not available for comment. ®

Updated to add
The Intel processor flaw is real. A PhD student at the systems
and network security group at Vrije Universiteit Amsterdam has
developed a proof-of-concept program that exploits the Chipzilla
flaw to read kernel memory from user mode:

View image on Twitter
View image on Twitter

brainsmoke
@brainsmoke
Bingo! #kpti #intelbug

6:28 AM - Jan 3, 2018
58 58 Replies 1,687 1,687 Retweets 2,362 2,362 likes
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The Register has also seen proof-of-concept exploit code that
leaks a tiny amount of kernel memory to user processes.

Finally, macOS has been patched to counter the chip design
blunder since version 10.13.2, according to operating system
kernel expert Alex Ionescu. And it appears 64-bit ARM Linux
kernels will also get a set of KAISER patches, completely
splitting the kernel and user spaces, to block attempts to
defeat KASLR. We'll be following up this week.

https://www.theregister.co.uk/2018/0...u_design_flaw/

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